Author Topic: Relavitistic impactors  (Read 1749 times)

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Offline Jetman123

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Relavitistic impactors
« on: June 29, 2012, 10:09:05 PM »
I realize that this will result in a "Cold War in space" style of story, but that can actually be interesting in the right circumstances. Assuming, of course, that diplomacy will get spread out a bit more in the future so that you can make backroom deals. I also assume that this would be a feature for much, much later and not on launch, but I can't help but bring it up.

Basically, the idea is, once you get an object going faster than about 0.3 c, the thing's makeup doesn't even make a difference anymore. A warhead is completely useless - it won't even have time to detonate before it hits it's target. Instead, the sheer momentum of impact will pretty much be capable of doing massive damage to a ship or a planet. If you have the time to unmolestedly accelerate something to massive speeds in deep space, you can pretty much guarantee being able to wreak destruction upon anything it hits.

You also have the time dilation effect meaning that you can't shoot the damned thing down, because it's always ahead of where it seems to be and is closing at such a fantastically fast rate that interceptions are pretty much useless. Though I'd be amused if some crafty player found a way to Macross Missile Massacre a curtain of continuously detonating nukes to obliterate it.

Possible uses include a reason for civilizations to expand in order to preemptively eliminate their rivals before they can build such a thing, requiring a special form of FTL intercept to destroy in deep space thus resulting in long-range extra-colony warfare, or the aforementioned "cold war in space" where a bunch of races have a dozen or so of these pointed at each other, ready to mutually annihilate one another if someone steps too far outside the rules of the game.

Just a random thought I had late at night for something in the far, far future. :D

http://www.projectrho.com/public_html/rocket/spacegunexotic.php#id--Relativistic_Weapons

http://www.projectrho.com/public_html/rocket/aliens.php#killingstar
 

Online Bremen

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Re: Relavitistic impactors
« Reply #1 on: June 29, 2012, 11:49:27 PM »
You also have the time dilation effect meaning that you can't shoot the damned thing down, because it's always ahead of where it seems to be and is closing at such a fantastically fast rate that interceptions are pretty much useless. Though I'd be amused if some crafty player found a way to Macross Missile Massacre a curtain of continuously detonating nukes to obliterate it.

Time dilation does not work like that. You might be thinking of lightspeed delay, but that could easily be compensated for and would exist even if the projectile was moving much slower.

As it happens this discussion has come up before in this forum, and never really reached much of a conclusion. It's generally agreed that defending a planet will be pretty much impossible unless there's some handwavium preventing/defending against long range kinetic projectiles.
 

Offline fcharton

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Re: Relavitistic impactors
« Reply #2 on: June 30, 2012, 08:46:28 AM »
Basically, the idea is, once you get an object going faster than about 0.3 c, the thing's makeup doesn't even make a difference anymore. A warhead is completely useless - it won't even have time to detonate before it hits it's target.

This happens way before relativistic speeds. Energy is proportional to the square of (relative) speed, which means even a relatively small projectile moving at a few percents of light speed will cause tremendous damage upon impact. This is not a far future problem, but a very real one of all space war simulations.

Most of the time, we tend to model war in space using a mix of blue water navies, and air wars. It sort of works "in our mind" because it looks like things we know, or read about. But it doesn't really stand the test of physics, because energy doesn"t scale (that's the square dependence above mentioned), and because the absence of gravity and friction makes priorities different...

Relativity comes on top of it, but at a later stage.

You also have the time dilation effect meaning that you can't shoot the damned thing down, because it's always ahead of where it seems to be and is closing at such a fantastically fast rate that interceptions are pretty much useless.

I don't think this is how "time dilation" works. In special relativity, time dilation is pretty much a "mathematical gotcha". Since the speed of light is constant in every frame, clocks can't "flow" everywhere at the same speed, hence the time dilation. But for one observer (either the missile or the target) time always flows at the same speed, and so long the missile closes in below light speed you can detect it before it reaches you, and react accordingly, if you detect it soon enough, and react fast enough.

Again, this exists in the non relativistics world : that's the reason why you can dodge a frisbee but not a bullet...

Francois

 

Offline Person012345

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Re: Relavitistic impactors
« Reply #3 on: June 30, 2012, 02:19:45 PM »
I don't think this is how "time dilation" works. In special relativity, time dilation is pretty much a "mathematical gotcha". Since the speed of light is constant in every frame, clocks can't "flow" everywhere at the same speed, hence the time dilation. But for one observer (either the missile or the target) time always flows at the same speed, and so long the missile closes in below light speed you can detect it before it reaches you, and react accordingly, if you detect it soon enough, and react fast enough.

Again, this exists in the non relativistics world : that's the reason why you can dodge a frisbee but not a bullet...

Francois


It's not just clocks though. The clock you mention would be ticking at the same speed to the people aboard the ship as it would normally (everything inside the ship would be slowed, yet appear to an observer on the ship as though it were proceeding as it always has, thus actual time inside the ship is slowed). I can explain this if necessary, if anyone is interested. Otherwise, the point has been made that what the OP said is not how time dilation works.
 

Online Bremen

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Re: Relavitistic impactors
« Reply #4 on: June 30, 2012, 04:20:24 PM »
It's not just clocks though. The clock you mention would be ticking at the same speed to the people aboard the ship as it would normally (everything inside the ship would be slowed, yet appear to an observer on the ship as though it were proceeding as it always has, thus actual time inside the ship is slowed). I can explain this if necessary, if anyone is interested. Otherwise, the point has been made that what the OP said is not how time dilation works.

Nah, on the each ship they see the clocks on the other ship as moving slow; time isn't slowed for anyone in their own reference frame.

Regardless, it shouldn't mess up targeting calculations unless the ship is currently undergoing acceleration, and even that can be compensated for.
 

Offline Person012345

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Re: Relavitistic impactors
« Reply #5 on: June 30, 2012, 04:48:27 PM »
Nah, on the each ship they see the clocks on the other ship as moving slow; time isn't slowed for anyone in their own reference frame.

Regardless, it shouldn't mess up targeting calculations unless the ship is currently undergoing acceleration, and even that can be compensated for.
That's what I was saying, they view their own clocks as perfectly normal. :)
 

Offline jseah

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Re: Relavitistic impactors
« Reply #6 on: June 30, 2012, 07:07:42 PM »
Start with this:
http://aurora2.pentarch.org/index.php/topic,4585.msg46785.html#msg46785

And read the ensuing discussion. 
 

Offline swarm_sadist

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Re: Relavitistic impactors
« Reply #7 on: July 09, 2012, 12:41:07 AM »
I'm curious about whether Einstein's kinetic energy formula is going to be used at high relativistic speeds, or if newton's simplified Ek=0.5mv^2.


RELATIVISTIC MOMENTUM
P= (Mass*Velocity)*(1-(v/c)^2)^-0.5

RELATIVISTIC ENERGY-KINETIC
Ek= mc^2 * (1-(v/c)^2)^-0.5 - mc^2
E(rest) = mc^2

This would nerf higher speeds as the mass of the object would exponentially increase the closer you got to the speed of light, requiring exponentially more power for very little gain in velocity. It would drastically increase the final energy and mass though (making AG sensors more effective against them?).
 

Offline UnLimiTeD

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Re: Relavitistic impactors
« Reply #8 on: July 10, 2012, 03:25:54 AM »
I think it's pretty unlikely for the first 50 or so game hours to ever reach such speeds, so I question the effort.
If Steve decides favorably about this, it will probably be simplified.
 

Offline procyon

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Re: Relavitistic impactors
« Reply #9 on: July 10, 2012, 05:22:33 AM »
Quote from: swarm_sadist link
I'm curious about whether Einstein's kinetic energy formula is going to be used at high relativistic speeds, or if newton's simplified Ek=0.5mv^2.

I'm with unlimited on this.
At 0.1c, the Lorentz factor is only one half a percent.  I am willing to ignore this in most calcs for a game.
Even at 0.5c (which I assume will be difficult to acheive period), it still only amounts to 15 percent of the original mass/apparent size/etc.  To make this something Steve can code without NASA wanting rights to the code - I am willing to even overlook a 15% fudge.  You don't even get a 100% change until 0.86c.  That is awful fast.
Steve is already fudging the fuel/accels for vector changes to a degree.  If I see lots of objects moving at 0.9c, then I might begin to have an issue with the giggle factor coming in.  But I just don't anticipate it.

And figuring it in for every object in relation to every other (as Lorentz only applies to something with relative motion, so it would be different for every single ship with a different location/velocity/vector) would probably take my computer a week to process a single one second increment....

You also have a part of your equation missing from your relative KE equation missing.
« Last Edit: July 10, 2012, 06:46:59 AM by procyon »
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Offline sloanjh

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Re: Relavitistic impactors
« Reply #10 on: July 10, 2012, 08:47:12 AM »
I'm with unlimited on this.
me too - ships aren't likely to be going fast enough for it to be significant.  The biggest reason to do it would be to prevent anyone from accellerating past the speed of light.
Quote
And figuring it in for every object in relation to every other (as Lorentz only applies to something with relative motion, so it would be different for every single ship with a different location/velocity/vector) would probably take my computer a week to process a single one second increment....
This shouldn't be necessary.  You just use the frame of the system as the standard frame (which is what's done anyway).  The easy thing to do would be to simply multiply deltaV costs by Gamma (the 1/sqrt(1-(v/c)^2)) factor, since deltaV is actualy (deltaP/restMass).  If P = restMass*V (Newtonian), then you get deltaV, but if P=(relativisticMass/restMass) you get a factor of Gamma.  Steve could also put individual clocks on each ship (for maintenance) and worsen reaction times by a factor of Gamma, but that's REALLY unlikely to be significant.
Quote
You also have a part of your equation missing from your relative KE equation missing.

I don't see anything wrong with it.  I did a double-take since he broke the rest mass out to get kinetic energy, but it looks like relativisticMass = restMass*Gamma to me, which is the correct expression.

John
 

Offline procyon

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Re: Relavitistic impactors
« Reply #11 on: July 13, 2012, 03:54:36 AM »
Gamma is the inverse of the square root of 1 less beta squared.

Haven't checked for edits, but the first time I looked it wasn't an inverse.

EDIT

Ok, went back and checked.  I see a -0.5 now.  That would be correct.

If it was there before and I simply missed it, my apologies.
« Last Edit: July 13, 2012, 05:40:37 AM by procyon »
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